1. Field of the Invention
The present invention is generally related to an idle setting mechanism for an internal combustion engine and, more specifically, to a means by which the speed of an engine can be accurately set to a very low magnitude which is slightly greater than the idle speed of the engine.
2. Description of the Prior Art
Many different types of internal combustion engines are well-known to those skilled in the art. Some internal combustion engines are used to provide the motive power for an outboard motor.
Most internal combustion engines provide an adjustment mechanism by which the idle speed of the engine can be set. Typically, an idle stop screw is used to define a position against which a throttle shaft is stopped from rotating in response to a spring when a manual force is removed from the throttle control. In other words, most engines are arranged to allow manual control of a throttle position so that the operating speed of the engine can be controlled. When the throttle control is set to its minimum position, a component attached to the throttle shaft rotates into contact with the stop screw and prevents further rotation of the throttle shaft. By adjusting the idle stop set screw, the engine speed at idle can be selected. This technique for determining the idle speed of an engine is very wellknown to those skilled in the art.
U.S. Pat. No. 4,902,448, which issued to Phillips on Feb. 20, 1990, describes a cam and idle speed adjustment for a marine engine. A marine carburetor is provided with an adjustment screw that is mounted on the throttle lever. It is engageable with the cam follower in such a manner that movement of the adjustment screw urges the cam follower into engagement with the surface of the throttle cam, at which point continued movement of the screw results in movement of the throttle lever so that the throttle plate may be adjusted to an idle position.
U.S. Pat. No 5,522,362, which issued to Motose on Jun. 4, 1996, discloses an idle control arrangement for an engine. The idle speed control arrangement embodies an idle air passage formed in a member of the induction system that is downstream of a throttle valve. The idle air passage is formed by a groove in a face of the member that is closed by engagement with another member of the induction system. The idle speed is controlled by controlling the flow of air to the idle passage from an atmospheric air inlet.
U.S. Pat. 4,784,096, which issued to Eberline on Nov. 15, 1988, describes a carburetor idle vent control. A carburetor for an internal combustion engine includes a supplemental fuel chamber and a fuel injector valve, manually or remotely actuated, to inject a charge of fuel into the fuel induction chamber of an associated engine and provide a continuing fuel supply to the engine, over and above the normal carburetor system, to allow the engine to reach a warm-up stage and prevent stalling until such stage is reached where normal carbonation can take over. Automatic refill and vent valves permit fuel recharge of the supplemental fuel chamber to ensure readiness of the chamber for starting at all times. An idle vent is arranged to be closed electrically or otherwise in order to facilitate the starting cycle.
U.S. Pat. No. 4,337,053, which issued to Stevens on Jun. 29, 1982, describes an idle adjustment control and sculptured twist grip throttle control handle for a marine propulsion device. The marine propulsion device comprises a marine propulsion unit including an engine and rotatably mounted propeller which is operatively connected to the engine. The engine has a throttle that is moveable between an idle position and an advanced position. A steering tiller is attached to the marine propulsion unit and a throttle grip is rotatably attached on the steering tiller. The throttle grip has an exterior gripping surface. A throttle linkage assembly is operatively connected to the throttle grip and the throttle from moving the throttle between the idle position and the advanced position in response to rotation of the throttle grip. An idle adjustment assembly is carried by the throttle grip and is operatively connected with the throttle linkage assembly for adjusting the low operational engine speed above the true idle of the engine. A portion of the idle adjustment assembly protrudes outwardly from the sides of the exterior gripping surface to permit the operator to operate the idle adjustment assembly without removing his or her hands from the throttle grip. The gripping surface of the throttle grip is contoured to include a first surface area spaced generally at a first radial distance from the rotational axis of the throttle grip and a second surface area which is faced angularly from the first surface area with respect to the rotational axis and generally at a second radial distance from the rotational axis which is different from the first distance. The operator is thus able to judge the approximate position of the throttle by visual and/or tactile reference to the contoured throttle grip.
U.S. Pat. No. 4,111,174, which issued to Fitzner et. al. on Sep. 5, 1978, discloses an admission system with idle speed governor apparatus. An alternator driven capacitive discharge ignition system includes a tachometer circuit which monitors the speed dependent repetition rate of discharge of an internal trigger power supply. The tachometer circuit causes the triggering threshold bias to be reduced below idle speed to electronically advance the timing. The ignition circuit includes a triggering threshold bias capacitor in series with the trigger pulse source and a trigger power supply having a pilot capacitor to alternately fire a pair of ignition silicone controlled rectifiers. The pilot capacitor is charged in series with an RC filter network of a "bucket" tachometer circuit to create a speed signal voltage proportional to engine speed with the pilot capacitor functioning as the bucket capacitor. The speed signal is a voltage which is positive relative to a common signal ground while the threshold bias capacitor voltage is negative to such signal ground. The speed signal is applied to the gate of a P-channel depletion mode junction field effect transistor (JFET). The source-to-drain channel of the transistor is connected in parallel with the threshold bias capacitor. Below the selected idle speed, the source-to-drain channel resistance provides a maximum bleed current to the threshold bias capacitor, thereby reducing the triggering threshold voltage and creating an automatic spark advance. As the engine speeds up, the source-to-drain resistance increases and virtually eliminates the bleed current at speeds slightly above idle.
With engines used for marine propulsion, such as outboard motors, it is sometimes desirable to operate the engines at very low speeds which are only slightly higher than the idle speed of the engine. For example, some fishermen desire to operate the outboard motor in gear but at a very low speed to allow a fishing technique that is referred to as trolling. During the trolling operation, the fisherman typically pulls a lure through the water behind the boat, and the speed of the lure moving through the water is generally determined by the speed of the boat. When used in this way, it is desirable that the outboard motor be operated at a very slow speed, but at a speed which is faster than normal idle. Usually, when a fisherman is trolling, the speed of the boat is manually controlled by continual adjustment of the handle grip on the tiller which serves as the throttle control. It is often very difficult to maintain a constant trolling speed in this manner.
It would, therefore, be highly desirable if a means could be provided to adjust the slow speed operation of an engine in a way that maintains a constant slow speed without the need for continual throttle adjustment.